The background of the invention will be discussed in two parts:
1. Field of the Invention
This invention relates to a dynamic braking circuit for a motor and more particularly for a universal motor having an armature and a series field winding.
2. Description of the Prior Art
Many circuits have been devised for controlling the braking rate for total brake time for alternating current and direct current motors. Generally, the braking characteristics desired are selected according to the particular application in which the motor is used.
One of the more conventional methods of dynamic braking of a motor is the utilization of a brake resistor, which can be fixed in value or adjustable. In the normal universal type motor the armature is connected in series with the field winding during operation of the motor. When braking is desired, a switch is actuated which reconfigures the circuit to disconnect the power source and connect the armature and field in series with a brake resistor. Other means have also been devised, some of which are shown in the following U.S. Pat. Nos. 2,507,918; 3,107,321; 3,297,930; 3,319,143; 3,504,257; 3,548,276; 3,568,018; 3,628,112; 3,678,353; 3,786,329; 3,811,080; 3,851,233; and 3,991,352.
Some of the braking systems devised as illustrated in the above patents apply to single phase AC motors, multi-phase AC motors and DC motors.
U.S. Pat. No. 3,548,276 is directed to a dynamic braking circuit for universal motor wherein a capacitor is charged during periods of motor energization, the energy so-stored being delivered to the motor windings after de-energization to assure that adequate magnetic flux is present after de-energization to place the motor in a generative mode. During switching to the braking condition the field winding is connected in phase reversed relation and in series with the rotor winding and a diode.
In centrifuge applications, a motor is used to rotate a centrifuge rotor having equal angularly spaced pivotable containers about the periphery thereof. In such applications, due to the large mass of the rotor, uniform or approximately linear braking is desirable within a minimum braking time.
It is accordingly an object of this invention to provide a new and improved dynamic brake circuit.
It is another object of this invention to provide a new and improved dynamic brake circuit for a universal motor having an armature with a series field winding.
It is a further object of this invention to provide a new and improved dynamic brake circuit providing deceleration in an optimal time with the rate of change of the deceleration rate being more even.